1. Field of the Invention
This application is based on patent application No. Hei 09-262561 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
FIG. 5 is a schematic structural diagram showing a conventional external resonator light source. In this figure, the numeral 1 indicates a semiconductor laser having two edge surfaces la,lb, wherein edge surface la is the reflecting surface and edge surface lb has a coating to prevent reflection formed thereto; 2 is a diffraction grating provided to the side of edge surface lb of semiconductor laser 1; 3 is a total reflecting lens which is provided perpendicular to light having the desired wavelength from among the light which is outgoing from diffraction grating 2, total reflecting mirror 3 reflecting this outgoing light and returning it to its source; 4 is an optical fiber provided to the side of edge surface la of semiconductor laser 1; 5 is a collimator lens provided between semiconductor laser 1 and diffraction grating 2; and 6 is a condenser provided between semiconductor laser 1 and optical fiber 4.
Of the light 7 generated in semiconductor laser 1, a portion of the light traveling to the edge surface la side of semiconductor laser 1 is reflected by edge surface la to become reflected light, while light which is not thus reflected passes through edge surface la. 
Conversely, of the light 7 generated in semiconductor laser 1, light 10, consisting of light traveling toward the edge surface 1b side of semiconductor laser 1 and the aforementioned reflected light, is not reflected by the antireflection film formed to edge surface 1b, but is outgoing from semiconductor laser 1. Collimator lens 5 then renders light 10 into parallel light, and incidents it on diffraction grating 2 so that diffraction occurs. Diffracted light 11 which has been diffracted by diffraction grating 2 is then dispersed at each wavelength, and travels out from diffraction grating 2.
Total reflecting mirror 3 is disposed so as to be perpendicular only to light having the desired wavelength from among diffracted light 11. Thus, light of the desired wavelength which is reflected by total reflecting mirror 3 returns along the same light path, is reflected by diffraction grating 2, and then passes through semiconductor laser 1 to again undergo reflection and transmission at edge surface la of semiconductor laser 1.
The wavelength oscillated by the laser at this time is determined according to an angle xcex1, formed by normal 12 of diffraction grating 2 and the light 10 that incidents on diffraction grating 2 from the semiconductor laser 1 side, and an angle xcex2, formed by normal 12 of diffraction grating 2 and the light 13 that reincidents on the diffraction grating from the total reflecting mirror 3 side.
In this way, a laser oscillation occurs in light 7 generated in semiconductor laser 1, by means of the resonator formed by edge surface 1a of semiconductor laser 1 and total reflecting mirror 3. The output of this laser oscillation is then output to the outside via edge surface 1a. Light 14 output to the outside via edge surface 1a of semiconductor laser 1 is condensed at condenser 6, incidents on optical fiber 4, and is then employed as a signal light.
However, conventional external resonator light sources are problematic in that laser light having the wavelength selected by a wavelength selecting element and light which is naturally emitted from the light amplifying element are output simultaneously. Thus, laser light 17 and natural emitted light 18 are simultaneously output as shown in FIG. 6, resulting in a deterioration in the purity of the wavelength of the light source.
For example, in the above described semiconductor laser 1, light 14 output to the outside via edge surface la includes both the laser light and natural emitted light generated inside semiconductor laser 1. As a result, it is typically the case that both laser light 17 and natural emitted light 18 are simultaneously measured in the light outgoing from an external resonator light source.
When an element other than a semiconductor laser is employed for the light amplifying element, a method may be employed in which laser oscillation is carried out by resonating the light generated from the light amplifying element using two reflecting surfaces. However, even in this case, natural emitted light generated by the light amplifying element is simultaneously measured, in addition to the light subjected to laser oscillation through selective oscillation by the wavelength selecting element.
Accordingly, as is the conventional practice, when employing a method in which the light directly emitted from the light amplifying element is output, both laser light and natural emitted light are simultaneously measured in the signal light. As a result, it has not been possible to avoid a deterioration in the wavelength purity of the light source.
The present invention was conceived in consideration of the above-described circumstances, and has as its objective the provision of an external resonator light source which, by removing natural emitted light, is able to send out only light which has an extremely high wavelength purity.
In order to resolve the problems described above, the present invention employs an external resonator light source of the following design.
Namely, the external resonator light source according to claim 1 is provided with a light amplifying element; a first light reflecting means disposed on one of the sides of the light amplifying element from which outgoing beams are emitted; a wavelength selecting element disposed on the other side of the light amplifying element from which outgoing beams are emitted; and a second light reflecting means which reflects and/or transmits outgoing light from the wavelength selecting element, and which, together with the first light reflecting means, forms a light resonator. This external resonator light source according to claim 1 employs the second light reflecting means to output outgoing light from the wavelength selecting element as transmitted light.
The external resonator light source is provided with a light coupling means on the light transmission side of the second light reflecting means for incidenting the transmitted light on the path of light transmission.
The external resonator light source is designed such that the first reflecting means serves as a reflecting surface provided on one of the edge surfaces of the light amplifying element from which outgoing beams are emitted.
Additionally, in this external resonator light source, the reflection coefficient of the first light reflecting means is set to be higher than the reflection coefficient of the second light reflecting means.
In the external resonator light source, the wavelength selecting element may be a diffraction grating.
In the external resonator light source, the wavelength selecting element may be a wavelength varying filter.
In the external resonator light source, the path of light transmission is either an optical fiber or an optical waveguide.
The present invention""s external resonator light source is provided with a light amplifying element; a first light reflecting means disposed on one of the sides of the light amplifying element from which outgoing beams are emitted; a wavelength selecting element disposed on the other side of the light amplifying element from which outgoing beams are emitted; and a second light reflecting means which reflects and/or transmits outgoing light from the wavelength selecting element, and which, together with the first light reflecting means, forms a light resonator; wherein light outgoing from the wavelength selecting element is output as transmitted light from the second light reflecting means. As a result, natural emitted light generated from the light amplifying element is removed by the wavelength selecting element, so that only light of the desired wavelength passes out therefrom, and only light of the desired wavelength which has been selected by the wavelength selecting element is transmitted from the second light reflecting means to be output as transmitted light. Thus, natural emitted light is removed, making it possible to send out light of the desired wavelength having an extremely high wavelength purity.
In the present invention""s external resonator light source, the first light reflecting means may have a high reflection coefficient, however, the reflection coefficient of the second light reflecting means is set low enough so that light of the desired wavelength selected by the wavelength selecting element is transmitted. In other words, it is desirable to set the reflection coefficient of the second light reflecting means to be lower than the reflection coefficient of the first light reflecting means.
This results in reduced loss in the light resonator formed by the first and second light reflecting means, making it possible to achieve a large light output with respect to the transmitted light.